The present invention relates generally to the formation of coatings on surfaces which are subjected to high temperature. More particularly, it relates to the formation of protective coatings on metal surfaces in which the coatings inhibit the flow of heat from ambient gases to the metal beneath the coating. These are known in the art as thermal barrier coatings.
It has been known for some time that it was possible to deposit coatings having relatively low thermal conductivity on the exterior of metal parts in jet engines. The object of depositing such coatings is to reduce or limit the flow of heat through the coatings to the engine parts and in this way to permit the temperature of the gas in contact with the coatings of low thermal conductivity to operate at higher temperatures. For example, it is known that plasma spray coatings of partially stabilized zirconia have been under development for many years for jet engine applications. Such coatings may have a composition of 8 weight percent yttrium oxide in a zirconium oxide base. Such coatings are known in the art as YSZ coatings or yttria stabilized zirconia coatings.
A major limitation of the use of this type of coating in jet engine applications is the tendency of such coatings to fail by spallation. One reason for the spallation is that generally coating materials, which might be considered suitable because of their low thermal conductivity, are found to be unsuitable because of their very low thermal expansion relative to the thermal expansion coefficient of the metal substrate on which they are deposited.
Some prior art has been developed which relates to the formation of protective coatings on the parts of gas turbine engines which operate at higher temperatures.
A U.S. Pat. No. 4,005,989 is directed toward the formation of an aluminide on the surface of a metal element and to the overcoating of the aluminide with MCrAlY where M is cobalt, nickel, or iron.
A U.S. Pat. No. 4,246,323 employs NiCrAlY in coating a metal element and then hot isostatically presses the coated element.
U.S. Pat. No. 4,774,149 which is assigned to the same assignee as the subject application deals with nickel base alloys for use in the hot section of gas turbine engines and to use of these alloys in deposited coatings or bonded claddings.
U.S. Pat. No. 3,869,779 deals with the coating of a metallic base with a ductile alloy layer that is oxidation resistant. An aluminide coating is then applied over the metallic alloy layer.
U.S. Pat. No. 4,371,570 discloses a substrate metal with an overlay coating based on iron, nickel or cobalt or mixtures thereof combined optionally with aluminum, yttrium and hafnium. A silicon-rich surface zone is produced at the surface of the overlay coating.
An article entitled "Production of Composite Structures by Low Pressure Plasma Deposition", by Siemers et al., was published in Ceramic Engineering and Science Proceedings, Vol. 6, No. 7-8 (July 1985). It describes the deposition of a variety of coatings on high temperature surfaces. Coatings containing both metal and oxide are described and illustrated.
A paper entitled "Survivability of Thermal Barrier Coatings", by H. Herman, appeared in Material Science and Engineering, Vol. 88 (1987) pages 69-74. This paper dealt with the use of ceramic oxide coatings in conjunction with MCrAlY-type intermediate bond coatings applied over superalloy substrates.
A paper entitled "Current Status of Thermal Barrier Coatings--An Overview", by Robert A. Miller, appeared in Surface and Coatings Technology, Vol. 30 (1987) pages 1-11.
A paper entitled "Thermal Barrier Coatings For Jet Engines Improvement", by G. John Kohner et al., appeared in Thin Solid Films, Vol. 119 (1984) pages 301-315.
These papers describe in detail some efforts which have been made in developing the technology of thermal barrier coatings and a variety of materials which have been employed in the study of such coatings.
The present invention relates to improvements which have been made in the coatings.